1. Field of the Invention
The present invention relates generally to improvements in data storage methods, and, more particularly, but not by way of limitation, to improvements in methods for acquiring data tracks in hard disk drives and initiating transfer of data to or from the tracks. 2. Brief Description of the Prior Art.
Computer peripherals generally include a device for storing data generated by the computer and a device which is increasingly being chosen for this purpose is a hard disk drive. Such a device is comprised of a stack of metal disks that are mounted on a spindle for rotation about their axes and a plurality of transducer heads that are supported by an actuator adjacent the stack so that the heads can be moved radially across the surfaces of the disks. The disks have magnetizable coatings so that concentric data tracks can be defined on their surfaces for storage of data in the form of magnetic transitions between consecutive segments of the tracks. The data is written to the tracks by passing currents, in one direction on one side of the transition and in the opposite direction on the other side of the transition, through a coil in the transducer head. Subsequently, the data can be read by utilizing the same coil to detect time varying magnetic fields that result from the passage of the transitions by the transducer heads.
The tracks on different disks are arranged in cylinders that are coaxial with the spindle and the drive has a servo system that is utilized both to seek to a selected cylinder and thereafter follow the tracks on the cylinder. A seek to a selected destination track is commonly made in accordance with a velocity demand profile that is stored in a microcomputer and the servo system includes a circuit that passes an electrical current through a coil, immersed in a magnetic field, on the actuator in relation to the difference in the actual radial velocities of the transducer heads across the disks and demand velocities called for by the velocity demand profile. The demand velocities at large distances from the destination track are large, to give rise to a large initial acceleration of the heads toward the destination track and decrease to zero, to decelerate the heads, as the destination track is approached.
The major advantage of the hard disk drive is that it is capable of storing a large amount of data and it is this storage capacity that has brought the hard disk drive into widespread use. Such storage capacity is achieved by a correspondingly high density of tracks on the disk; for example, a disk will typically contain about 1600 tracks per inch. However, such track densities exact a price. Unless the heads are closely aligned with tracks during the reading of data, excessive read errors may occur and, worse, during writing, data may be written to an adjacent track to destroy previously stored data. While the latter can be, and is, prevented by defining "off track" thresholds about the tracks beyond which writing will not take place, the cure can give rise to so-called write faults; that is, stoppages after writing has commenced, should the heads subsequently become displaced from track a distance sufficient to reach the "off track" threshold. Thus, while data on adjacent tracks is not destroyed, a second important consideration in the storage and retrieval of data, throughput; that is, the average rate at which data can be stored and retrieved by a computer using the hard disk drive, is adversely affected.
As will be clear to those skilled in the art, the result of a write fault will be that the disks must be rotated through a full revolution before the location to which the data is to be written again comes into alignment with the head that is to do the writing. Typically, the time for a complete revolution of the disks is about 16 milliseconds, a time that is very long by computer standards. Thus, a write fault can greatly increase the time required to store a particular document that has been generated by a computer and, consequently, reduce the throughput of the hard disk drive. Accordingly, write faults have been avoided in the prior art in a manner that will now be described.
As a read/write head approaches a track, it will cross an "on track" threshold at a selected distance from the track and an indication of the passage of the "on track" threshold is transmitted to the microcomputer that controls the operation of the hard disk drive and, in particular, determines when writing is to commence after passage of the head by the "on track" threshold. If, at the time of the passage, the velocity of the head is excessive, the head may overshoot the track by an amount that will cause it to pass the "off track" threshold on the other side of the track. At the other extreme, the velocity of the head may be so low that the head will stall prior to reaching the track or, for that matter, prior to reaching the "on track" threshold defined for the track. In the latter case, the microprocessor is programmed to make a transition from track seeking to a track following mode of operation, in which fine control circuitry is utilized to maintain the alignment of the head with the track, at a preselected time after the head enters a selected fine control region defined about the track so that the fine control circuitry can pull the head to the track. However, if the head is greatly displaced from the track when the transition to track following occurs, the fine control circuitry may exert large forces on the head that will again result in an overshoot of the track by an amount that will carry it beyond the "off track" threshold. Thus, in either case, the head may cross the "off track" threshold after crossing the "on track" threshold so that the initiation of writing must be delayed, if a write fault is to be avoided, until the head has settled upon the track sufficiently for all danger of subsequent crossing of an "off track" threshold to have passed. In the prior art, this danger is avoided by selecting a delay time, following passage of the "on track" threshold, that must elapse before writing commences and such delay time is selected, for all seeks, that will prevent a write fault for the worst case of either overshoot or stall.
The problem with using a "worst case" delay time before initiating reading or writing is that the throughput is unnecessarily diminished. Most seeks will result in stable track following considerably before the "worst case" delay time has elapsed so that use of such delay time can have a serious adverse affect on the time that elapses between the start of movement of the head toward a destination track and the time that reading or writing is commenced. As a result, until the present invention, lower than desired throughputs have generally had to be accepted. If a short delay time is selected, write faults will occur to lower throughput; longer delay times directly affect the throughput by unnecessarily prolonging the time for commencement of reading and writing until long after stable track following has been achieved.